Recordings on a magnetic disk can be performed either longitudinally or perpendicularly. In longitudinal recording, information is stored within or parallel to the plane of the magnetic disk. In perpendicular recording, information is stored perpendicular to the plane. Longitudinal recording products have been commercially available for some time; new perpendicular recording products are being developed because of the potential for much higher storage capacity compared to longitudinal recording.
A signal associated with a magnetic disk read channel may have a number of defects, such as offset (e.g., a constant voltage added to the signal), gain (e.g., a constant multiplied by the signal), or magneto-resistive asymmetry (MRA). MRA refers to distortion that results from a magneto-resistive read head operating in a nonlinear region of a magnetic field. Typically, feedback loops are used to handle offset, gain, and MRA. Existing offset loops and MRA loops are coupled, in that an offset error will cause the MRA loop to diverge. Perpendicular recording signals tend to have higher offset. Therefore, improved techniques for handling MRA would be desirable.